Maternal anti-KEL IgG crosses the placenta and binds to KEL RBCs of pups. (A) RBCs from KEL-positive and KEL-negative pups born to mothers immunized through pregnancy were evaluated for bound anti-KEL IgG by flow cytometric cross matching. (B) Serum from these same animals was evaluated for anti-KEL IgG in a direct antiglobulin test. Results are representative of 3 independent breeding experiments (n = 20 pups). *P < .05. (C) Anti-KEL subtypes in the serum of a representative alloimmunized mother compared with the serum of her KEL-negative pups. (D) Newborn liver cells from a representative KEL-negative and pale KEL-positive pup born to an alloimmunized mother, quantitated by Trucount beads (left column) or stained with anti-mouse Igs (right column). (E) Total number of pups and percentage of KEL-positive pups was evaluated in first and third litters of MuMT females bred with KEL males (n = 8 total pregnancies). P is not significant. No anti-KEL antibodies were detected in any MuMT female; error bars indicate standard deviation.